I can't say precisely with simplefit; though in my experience, as long as you eat a LOT of protein, say 1g/pound of body weight, plus a bit more calories than your body normally needs, and if you exercise consistently, you will gain muscle. Gained 20 pounds that way in the last 2 years and believe me it's tough for me!

EDIT : I feel it important to say that I primarily lifted weights during the time I gained that weight. However, I don't see any reason why simplefit wouldn't work just as well. It might be just a little slower, but the strength and endurance is all functional; you'll notice a difference in your everyday movements much more than with dumbbells strength workouts IMO.

Nutrition is 80% of the results and exercice 20%, but both need to be there for results to happen!

I can say that from my experience if you eat the calories the gainz will come, even if all you do is simplefit. Now you can definitely put on weight with paleo and crossfit as well or lose it all depends on how you eat and train.